Monitoring the hydrologic cycle

What is the hydrologic cycle?What is the hydrologic cycle?

The hydrologic or water cycle is the movement of water on, above, below, and through the earth’s surface.

Why do scientists measure the Why do scientists measure the hydrologic cycle?hydrologic cycle?

Weather forecasting Water use forecasting Understand climate trends Flood preparation Hydrologic modeling

Why is it important for scientists to Why is it important for scientists to measure the hydrologic cycle?measure the hydrologic cycle?

A rain gauge measures liquid and solid precipitation

Precipitation measurementsPrecipitation measurementsPrecipitation is the general name given for any form of condensed water that falls to the earth’s surface.

Snow depth sensors use sound to record snow depth

Snow pillow records weight of snow

Measuring StreamflowMeasuring Streamflow

Flumes make the stream fit into a known shape.

Runoff is the movement of water over the land surface.

Stream flow is the flow of surface water runoff contained in a stream channel.

Water depth sensors measure the height of water.

Measuring soil moistureMeasuring soil moisture

Water that does not flow to the stream channel moves through the soil and/or rock through a process called infiltration. This water is known as groundwater.

Soil moisture sensors buried at different depths underground.

A series of soil moisture sensors buried underground to understand water uptake by tree roots.

c

Measuring Measuring evapotranspirationevapotranspiration

The movement of water from the liquid phase to the gas phase occurs by evaporation and/or transpiration.

Evaporation is the change of water from a liquid to a gas by heating.

Transpiration is water released from plants.

Combined, this process is called evapotranspiration.

Sap flow

Flux tower instruments

Measuring evapotranspirationMeasuring evapotranspiration

The eddy-covariance flux tower measures water vapor from the surrounding forest. The tower measures: - wind speed and direction- CO2 and H2O content- air temperature - relative humidity- solar radiation

An evapotranspiration dome measures evaporation and transpiration coming off of soil and plant matter.

Weather stationsWeather stations

This weather station is used to

measure:

- air temperature- wind speed

- relative humidity

- solar radiation - precipitation- snow depth

Collecting and retrieving dataCollecting and retrieving data An immense amount of data are collected

each year. Instruments are located in remote areas,

making it hard to collect data on foot during the winter

Data stored on a datalogger and can be

downloaded with a laptop

Collecting and retrieving dataCollecting and retrieving data

The wireless embedded sensor network (WSN) was developed to make it easier to collect data remotely.

Data sent to satellites can be

retrieved remotely.Small radios send data from one node to the next, and end at the base station at the flux tower.

Powering instrumentsPowering instruments

A series of solar panels and

batteries power all the

instruments within the basin.

Measuring the hydrologic cycle componentsMeasuring the hydrologic cycle components

Many different electronic instruments are available to measure the water cycle.

People use data from these instruments to understand many aspects of environmental science.

Scientists need to measure the water cycle in order to determine how much fresh water is stored in the mountains.

MEAN AREAL DEPTH OF PRECIPITATION (Pave)

Point rainfall—It is the rainfall at a single station. For small areas less than 50 km2, point

rainfall may be taken as the average depth over the area.

In large areas, there will be a network of rain-gauge stations.

As the rainfall over a large area is not uniform, the average depth of rainfall over the area is determined by one of the following three methods:

average method(i) Arithmetic average method—It is obtained

by simply averaging arithmetically theamounts of rainfall at the individual

rain- gauge stations in the area, i.e.,

where P ave = average depth of rainfall over the areaΣP1 = sum of rainfall amounts at individual rain-gauge stations n = number of rain-gauge stations in the area

average method This method is fast and simple and yields good

estimates in flat country if the gauges are uniformly distributed and the rainfall at different stations do not vary very widely from the mean.

Limitations can be partially overcome if topographic influences and aerialrepresentation are considered in the selection of gauge sites.

Thiessen method(ii) Thiessen polygon method—This methodattempts to allow for non-uniform distribution of gauges by providing a weighting factor for each gauge.The stations are plotted on a base map and are connected by straight lines. Perpendicular bisectors are drawn to the straight lines, joining adjacent stations to form polygons, known as Thiessen polygons

Thiessen polygons

Thiessen polygon

Limitations of Thiessen Method Thiessen method is it is dependent on a

good network of representative rain gauges.

It is not particularly good for mountainous areas since altitudinal effects are not allowed for the areal coefficients

It is not useful for deriving areal rainfall from intense local storms

Isohyetal method(iii) The isohyetal method—In this method,

the point rainfalls are plotted on a suitable base map and the

lines of equal rainfall (isohyets) are drawn giving

consideration to orographic effects and storm morphology

Isohyetal method

Isohyetal method The average rainfall between the

successive isohyets taken as the average of the two isohyetal values (pi)are weighted with the area between the isohyets (ai), added up and divided by the total area (A) which gives the average depth of rainfall over the entire basin, i.e.

Aap

P iiave

Summary

Advantage of Isohyetal and Thiessen Methods Use of both outside and inside stations

located near the boundary The influence of outside stations

diminishes as their distance from the boundary increases

However, in Arithmetic mean method, every inside station has equal weight regardless of its location

Intensity-Duration-Frequency Curves (IDF)

In general, the higher the intensity of rainfall the shorter the duration it will take to rain

IDF relationship is important for engineers in designing hydraulic structures

IDF consists of a family of curves with each curve representing a certain frequency with change in intensity with respect to time (duration of storm)

Duration, tr (min)0 15 30 45 60 75 90 105 120 135 150 165 180

Inte

nsity

, i (m

m/h

r)

0

50

100

150

200

250

300

F requency100-year50-year25-year10-year5-year2-year

District 1

i = A/(B+tr)m

Frequency A B m2-year 1581 12.98 0.855-year 1995 14.19 0.8410-year 1874 13.14 0.8025-year 2175 13.49 0.7950-year 2121 12.64 0.76100-year 2603 13.71

Temporal DistributionsUnderstanding Temporal Distributions is very important,as this greatly affects runoff timing and volumes.

Temporal DistributionsTemporal Distributions Precipitation is a continuous process. Intensities vary depending on amount and

duration Gages record data at intervals - 10 min., 15 min.,

1 hour, 24 hour, etc.... Models may use the data at 1-hour, 6-hour, etc... Must either aggregate or disaggregate

precipitation amounts.... i.e. Combine 1 hour values into a 6-hour value...

Not a problem! Or... Break a 24-hour value into 6 hour values... Much more difficult!

Understanding IntensitiesTime Depth (inches Intensity (inches/hr)1 min 1.23 73.8

15 min 7.8 31.22 hrs 10 min 19 8.8

15 hrs 34.5 2.324 hrs 46 1.948 hrs 65.8 1.47 days 131 0.81 year 1042 N/A

Intensities & Durations A 5-minute recording gauge Recorded a storm for 40

minutes Calculate:

Total Rainfall Cumulative Rainfall Curve Max. 5,10, & 30 minute

intensities The average intensity

Time (min) Amount (mm)5 1.810 6.7215 15.620 6.9625 6.1230 11.6435 5.7640 1.56

The Data

Solutions• Total rainfall – simply sum the precipitation values:

56.16 mm or 2.21 inches

• Cumulative data is shown and plotted below:

Time (min) Amount (mm) Cumulative5 1.8 1.810 6.72 8.5215 15.6 24.1220 6.96 31.0825 6.12 37.230 11.64 48.8435 5.76 54.640 1.56 56.16

Solutions, cont….• The maximum 5 minute intensity was 15.6 mm between 10-15

minutes at 187.2 mm/hr or 7.3 inches/hr. This is illustrated in the data below:

Time (min) Amount (mm) Cumulative hourly intensities (mm/hr)5 1.8 1.8 21.610 6.72 8.52 80.6415 15.6 24.12 187.220 6.96 31.08 83.5225 6.12 37.2 73.4430 11.64 48.84 139.6835 5.76 54.6 69.1240 1.56 56.16 18.72

56.16

Solutions, cont…• The maximum 10 minute intensity was found by aggregating

sequential 5-minute periods. The maximum 10-minute intensity is illustrated below, between 10-20 minutes with 22.56 mm or 135.36 mm/hr or 5.29 inches/hr.

Time (min) Amount (mm) 10-minute blocks Intensities (mm/hr)inches/hr5 1.8 0 0 010 6.72 8.52 51.12 1.99687515 15.6 22.32 133.92 5.2312520 6.96 22.56 135.36 5.287525 6.12 13.08 78.48 3.06562530 11.64 17.76 106.56 4.162535 5.76 17.4 104.4 4.07812540 1.56 7.32 43.92 1.715625

56.16

Solutions, cont…• The maximum 30 minute intensity was found by aggregating

sequential 5-minute periods. The maximum 30-minute intensity is illustrated below, between 5-35 minutes with 52.8 mm or 105.6 mm/hr or 4.125 inches/hr.

Time (min) Amount (mm) 30-minute blocks Intensities (mm/hr) inches/hr5 1.8 0 0 010 6.72 0 0 015 15.6 0 0 020 6.96 0 0 025 6.12 0 0 030 11.64 48.84 97.68 3.81562535 5.76 52.8 105.6 4.12540 1.56 47.64 95.28 3.721875

56.16

Solutions, cont…The total rainfall was 56.16 mm over a duration of 40 minutes for an average intensity of 84.24 mm/hr or 3.29 inches/hr. In summary:Minutes mm/hr

5 187.210 135.3615202530 105.63540 84.24